1. Field of the Invention
The present invention relates to a hard disk drive. More particularly, the present invention relates to a method of determining operating characteristics of a read/write head of the disk drive, and to a method of controlling the elevation of the head using the characteristics.
2. Description of the Related Art
A hard disk drive is a data storage device in which data is read from and recorded onto a disk using a magnetic read/write head. More specifically, bits of the data are recorded onto and read from respective tracks on a magnetic recoding surface of the disk. To this end, the magnetic head is positioned over and in alignment with a desired track of the disk during a read/write operation in which the disk is rotated. Today, there is an ongoing demand for hard disk drives that are lighter and yet have higher and higher data storage capacities. Thus, the capacity of the magnetic disk of a hard disk drive, namely the number of bits per inch (BPI) and tracks per inch (TPI), is being increased to meet such a demand. Accordingly, developments in hard disk drives are also aimed at reducing the height at which the magnetic head is floated above the recording surface of the disk during a read/write operation, and increasing the frequency at which the bits of data can be read and recorded.
FIG. 1 shows a conventional hard disk drive 100. The conventional hard disk drive 100 includes at least one disk 112, a spindle motor 114 for rotating the disk, and a magnetic read/write head 120 for recording data onto and reading data from the disk 112. In particular, the head 120 reads or records information from or onto the disk 112 by detecting a magnetic field produced by the recording surface of the disk 112 or by magnetizing the surface of the disk 112. And, although FIG. 1 shows the read/write head 120 of the hard disk drive 100 as a single component, the read/write head usually includes a recording element for magnetizing the disk 112 and a separate reading element for detecting the magnetic field of the disk 112.
In any case, the hard disk drive 100 is designed such that the read/write head 120 and the disk 112 act as an air bearing. In this respect, the head 120 is coupled to a suspension 122 and is supported by the suspension such that the head 120 can move vertically relative the disk 112. Also, the head 120 has an air bearing surface that faces the disk 11. The suspension 122, in turn, is attached to an actuator 124 having a voice coil 126. The actuator 124 is mounted to a pivot bearing 132 so as to be rotatable about a central axis of the pivot bearing 132.
The voice coil 126 is disposed adjacent a magnetic assembly 130. Together, the voice coil 126 and the magnetic assembly 130 make up a voice coil motor for rotating the actuator 124 about the axis of the pivot bearing 132. More specifically, current supplied to the voice coil 126, within the magnetic field generated by the magnetic assembly 130, produces torque that rotates the actuator 124 about the axis of the pivot bearing 132. The rotation of the actuator 124 moves the head 120 across the surface of the disk 112. At this time, the rotation of the disk 112 by the spindle motor 114 induces air to flow between the rotating surface of the disk 112 and a head slider. The head slider is integral with the suspension and includes the air bearing surface and the read/write head 120. As a result, the read/write head 120 is elevated above the surface of the disk 112 as biased towards the disk 112 by the suspension.
FIG. 2 illustrates flying on demand (FOD) technology 220 that has recently been used for controlling the elevation of the read/write head, i.e., the height at which the head floats above the surface of the disk during a read/write operation. Referring to FIG. 2, the head 230 is an integral part of a head slider 220 and includes a recording element 236, a reading element 232, and an FOD heater 234 located between the recording element 236 and the reading element 232. The FOD heater 234 may comprise a pole at the periphery of the head, and a resistive heating element in the form of a coil wound around the pole. An air cushion is formed between the air bearing surface of the head slider 220 and the surface 210 of a disk when the disk is rotated. As a result, the head 230 floats or hovers above the surface 210 of the disk in a state in which a predetermined clearance is maintained between the head 230 and the disk. Reference numeral 212 indicates variations in the height of the surface disk surface 210. However, the head slider 220 is supported so that the predetermined clearance is maintained despite such variations.
Assuming that the target (desired) clearance between the head 230 and the surface 210 of the disk is A+B+C, the elevation to which a particular head 230 will be raised above the surface 210 of the disk may be as small as B+C due to mechanical tolerances or to other characteristics of the air bearing created by the head 230 and the rotating surface of the disk. In addition, the elevation may be reduced to as low as C due to environmental conditions such as temperature changes or changes in atmospheric pressure. In this case, though, the elevation of the head 230 can be controlled by operating the FOD heater 234 so that a sufficient clearance is maintained between the head 230 and the surface 210 of the disk during a read/write operation.
That is, the FOD technology 220 employs a heater to control the clearance between the disk and the head. Thus, the FOD technology 220 can be employed to ensure that all heads will float at the same height above the surface of a disk rotating a certain speed. More specifically, FOD technology ensures a uniform head disk interface (HDI) between the head and the disk and thereby minimizes the bit error rate (BER).
A burn-in process is used to determine certain physical characteristics of the read/write heads of hard disk drives which are mass-produced, i.e., are manufactured using the same process under identical environmental conditions (e.g., temperature, humidity, and pressure). Also, the burn-in process is used to determine the data storage capacity of the disks of the hard disk drives. The results of the burn-in process are then incorporated into the hard disk drives in an attempt to ensure that a sufficient clearance is maintained between the heads and the disks of the hard disk drives. Specifically, the results of the burn-in process are used to control the power circuits that supply current to the heaters of the FOD technology.
FIG. 3 illustrates a conventional method of correlating the elevation of a read/write head of a hard disk drive (represented in the figure as FH or “flying height”) to temperature using a burn-in process. The burn-in process begins by loading a plurality of hard disk drives into a burn-in chamber. Then, the FH of a read/write head of each hard disk drive is measured at a normal temperature (e.g., 25° C.) in the burn-in chamber (S310).
Next, one of the hard disk drives is selected (S320). Subsequently, the FH of the head of the selected hard disk dive is measured at each of several test temperatures (S330). For example, the FH of the head is measured at a low temperature (e.g., 0° C.), at the normal temperature (e.g., 25° C.), and at a high temperature (e.g., 55° C.) in the burn-in chamber.
An equation is formulated using values of the FH of the head of the selected hard disk drive measured at each test temperature (S340). The equation represents the FH of the head as a function of temperature. The equation may be represented as a first degree polynomial function. That is, the correlation between temperature and the flying height of the head of the selected hard disk drive may be established in the form of a linear relationship.
Finally, the flying height of the head of each of the rest of the hard disk drives at each test temperatures is estimated using the equation (S350). More specifically, the flying height of each head at the normal temperature is actually measured. Then, the linear relationship between FH and temperature, as determined based on the selected head, is used to estimate the FH of the head of each of the rest of the hard disk drives at the low and high temperatures. Such data, representing a correlation between FH and temperature, is then stored the hard disk drives.
FIG. 4A is a graph illustrating the results obtained using the conventional method. FIG. 4B, on the other hand, illustrates results obtained by the present inventors when the FH of all of the heads of the hard disk drives was actually measured at each of the test temperatures.
In FIG. 4A, the solid line represents the linear relationship between the FH of the head of the selected hard disk drive (hd1) and the temperatures at which the FH was measured. That is, the FH of the head hd1 as a function of temperature is represented as a line having a certain slope. Therefore, in the conventional method, linear polynomial functions, representing the FH of the heads of the rest of the hard disk drives (h2, h3) as a function of temperature, are each formulated based on the assumption that the slopes of the functions are identical to that of the equation formulated using the head hd1.
However, as shown in FIG. 4B, the estimates of the FH of the heads of each of the rest of the hard disk drives differ from the actual FH. That is, the slope of the linear function applicable to the head of the selected hard disk drive is not applicable to the heads of the other hard disk drives. This is due to the fact that even when the corresponding parts of the hard disk drives are manufactured under identical conditions and using identical processes, the parts have different characteristics as the result of, for example, slight variations in the material used to manufacture the parts.